Quadrupole mass spectrometers are a type of mass spectrometer in which a quadrupole mass filter is used for separating ions according to their mass-to-charge ratios. FIG. 6 shows a schematic configuration of a quadrupole mass spectrometer. Various kinds of ions produced in an ion source 1 are introduced through an ion transport optical system (not shown) into a quadrupole mass filter 2 composed of four rod electrodes 2a, 2b, 2c and 2d. Voltages±(U+V cos ωt) produced by superimposing radio-frequency (RF) voltages±V cos ωt on direct-current (DC) voltages±U are applied from a quadrupole power source 4 to the four rod electrodes 2a-2d. Only the ions having a specific mass-to-charge ratio corresponding to those voltages are selectively allowed to pass through the quadrupole mass filter 2. The ions which have passed through are detected by a detector 3, which acquires a detection signal corresponding to the amount of ions.
For example, when a scan measurement over a predetermined range of mass-to-charge ratios is performed, a controller 5 operates the quadrupole power source 4 so that the amplitude value V of the RF voltage V cos ωt and the value U of the DC voltage independently change while maintaining a specific relationship. By this control, the mass-to-charge ratio of the ions passing through the quadrupole mass filter 2 is continuously varied over a predetermined range of mass-to-charge ratios. Based on the detection signals acquired by the detector 3 during this scan, a data processor 6 creates a mass spectrum with the horizontal axis indicating the mass-to-charge ratio and the vertical axis indicating the ion intensity.
FIG. 7 is a schematic block diagram of a commonly used conventional quadrupole power source 4 (see Patent Documents 1 and 3). Coils 10 and 12 with inductance L and capacitors 11 and 13 with capacitance C′ are connected to the output of the quadrupole power source 4. The capacitance C in the rod electrodes 2a-2d is composed of the capacitances C′ of the capacitors 11 and 13 combined with the stray capacitance of the rod electrodes 2a-2d. The serial circuit of the combined capacitance C and the aforementioned inductance L functions as an LC resonance circuit. A resonance in this LC resonance circuit produces an RF voltage, which is to be superimposed on the DC voltage and applied to the rod electrodes 2a-2d. For example, the frequency of the RF voltage produced by the quadrupole power source 4 and supplied into the LC resonance circuit is f=1.2 MHz.
The condition for the resonance in the LC resonance circuit is f=1/(2π√{square root over (LC)}). There are the following two methods for satisfying this condition and creating a resonance: (1) the frequency f of the supplied RF voltage is fixed, and either the inductance of the coils 10 and 12 or the capacitance of the capacitors 11 and 13 is adjusted to tune the circuit and create an LC resonance; or (2) the inductance of the coils 10 and 12 as well as the capacitance of the capacitors 11 and 13 are fixed, and the frequency f of the supplied RF voltage is adjusted to tune the circuit and create an LC resonance. Method (1) has the problem that it requires expensive components for accurately varying the inductance of the coils 10 and 12 or the capacitance of the capacitors 11 and 13, and that it is in some cases difficult to ensure a stable performance due to a variation in the characteristics of the components. Therefore, in many cases, the frequency-variable tuning method as described in (2) is used. However, a quadrupole power source using the conventional frequency-variable tuning method has the following problem.
FIG. 8 shows the circuit configuration of a quadrupole power source 4 in which a commonly used conventional frequency-variable tuning method is adopted (see Patent Documents 1 and 2). In this circuit, a wave detector section 4D, which includes a diode bridge rectifier circuit 401 as well as detecting capacitors 402 and 403, detects the voltage value of the RF voltage applied to the quadrupole mass filter 2 (this value is hereinafter called the “V voltage”). The detection output is converted into a DC voltage and is fed back to an RF power supply section 4A and a DC power supply section 4B via a detection gain adjuster section 4C. The detection gain adjuster section 4C includes a V-voltage detecting resistor 404, a V-voltage adjusting amplifier 405 and a V-voltage adjusting variable resistor 406. The RF power supply section 4A includes a buffer amplifier 407, an m/z-axis adjusting variable resistor 408, a V-voltage comparing amplifier 409, a multiplier 410, an RF voltage signal generator 411, a buffer amplifier 412, a drive circuit 413 and an RF transformer 414. The DC power supply section 4B includes an inverting amplifier 415, a positive DC voltage amplifier 416 and a negative DC voltage amplifier 417.
The frequency f of the RF voltage supplied from the secondary coil of the RF transformer 414 to the LC resonance circuit including the quadrupole mass filter 2 is determined by the frequency of the rectangular signal generated by the RF voltage signal generator 411. The voltage value of that RF voltage in turn is determined by the voltage given from the V-voltage comparing amplifier 409 to the multiplier 410. The output voltage of the V-voltage comparing amplifier 409 depends on the detection output fed back from the wave detector section 4D, the power supply controlling voltage (Qcont) corresponding to the target mass-to-charge ratio given from the controller 5, the adjusting positions of the V-voltage adjusting variable resistor 406 and the m/x-axis adjusting variable resistor 408, and other factors.
The V-voltage adjusting variable resistor 406 has the function of adjusting the gain for amplifying the detection output fed back from the wave detector section 4D. A detection output voltage is amplified by the V-voltage adjusting amplifier 405 with the gain set by this resistor 406 and sent to a comparator for setting the V voltage, which consists of the m/z-axis adjusting variable resistor 408 and the V-voltage comparing amplifier 409, as well as to the DC power supply section 4B. The comparator for setting the V voltage, which consists of the m/z-axis adjusting variable resistor 408 and the V-voltage comparing amplifier 409, has the function of comparing the detection output after the gain adjustment with the power supply controlling voltage and determining the multiplier factor (or as it were, gain) of the multiplier 410 according to the comparison result.
The circuit of the quadrupole power source 4 operates in such a manner that a V-voltage monitoring voltage Vmon, which is the output of the V-voltage adjusting amplifier 405, is constantly maintained at the same level when the power supply controlling voltage Qcont is constant. Accordingly, the following relationships hold true.
      [          V      ⁢              -            ⁢      voltage      ⁢                          ⁢      monitoring      ⁢                          ⁢      voltage      ⁢                          ⁢      Vmon        ]    ∝                         [                  V          ⁢                      -                    ⁢          voltage          ⁢                                          ⁢          detecting          ⁢                                          ⁢          voltage          ⁢                                          ⁢          Vdet                ]            =                                             [                          Current              ⁢                                                          ⁢              i              ⁢                                                          ⁢              passing              ⁢                                                          ⁢              through              ⁢                                                          ⁢              the              ⁢                                                          ⁢              detecting              ⁢                                                          ⁢              capacitor              ⁢                                                          ⁢              402              ⁢                                                          ⁢              or              ⁢                                                          ⁢              403                        ]                    ×                                                                 [                                  Resistance                  ⁢                                                                          ⁢                  R                  ⁢                                                                          ⁢                  of                  ⁢                                                                          ⁢                  the                  ⁢                                                                          ⁢                  V                  ⁢                                      -                                    ⁢                  voltage                  ⁢                                                                          ⁢                  detecting                  ⁢                                                                          ⁢                  resistor                  ⁢                                                                          ⁢                  404                                ]                            ∝                                                                                     [                                          V                      ⁢                                                                                          ⁢                      voltage                                        ]                                    ×                  2                  ⁢                  π                  ⁢                                                                          ⁢                  f                  ×                                      [                                          Capacitance                      ⁢                                                                                          ⁢                      C                      ⁢                                                                                          ⁢                      of                      ⁢                                                                                          ⁢                      the                      ⁢                                                                                          ⁢                      detecting                      ⁢                                                                                          ⁢                      capacitor                      ⁢                                                                                          ⁢                      402                      ⁢                                                                                          ⁢                      or                      ⁢                                                                                          ⁢                      403                                        ]                                    ×                                      [                                                                  Resistance                        ⁢                                                                                                  ⁢                        R                        ⁢                                                                                                  ⁢                        of                        ⁢                                                                                                  ⁢                        the                        ⁢                                                                                                  ⁢                                                                                                         V                            ⁢                                                          -                                                        ⁢                            voltage                            ⁢                                                                                                                  ⁢                            detecting                            ⁢                                                                                                                  ⁢                            resistor                                                    ]                                                                    ∝                                                                        [                                                      V                            ⁢                                                                                                                  ⁢                            voltage                                                    ]                                                ·                        f                                                                                                                                    
That is to say, in the circuit of the quadrupole power source 4 shown in FIG. 8, the V voltage is inversely proportional to the frequency f. Therefore, for example, the higher frequency f is, the lower the V voltage is. This means that, in the frequency-variable tuning method, the V voltage changes when the frequency of the RF voltage is changed for the purpose of tuning. For example, a 0.2% increase in the frequency f (from 1.2 MHz to 1.20024 MHz) causes a 0.2% decrease in the V voltage. This causes a change in the UN ratio, despite the fact that this ratio should be maintained at the same value. As a result, the mass-resolving power becomes higher (and the sensitivity becomes lower) than it should be within a high mass-to-charge ratio range.
FIGS. 9A and 9B are examples of peak profiles actually measured at a plurality of mass-to-charge ratios for a standard sample, where FIG. 9A shows the result obtained when the frequency f was optimally adjusted to 1.2 MHz, and FIG. 9B shows the result obtained when the frequency f was slightly increased from the state of FIG. 9A to 1.20024 MHz (without voltage adjustment). A comparison of FIGS. 9A and 9B demonstrates that the peaks in FIG. 9B have smaller half-value widths and lower peak values within a range where the mass-to-charge ratio is high. This means that the mass-resolving power is improved while the detection sensitivity is lowered.
According to the Mathieu equation which is used for analyzing the stability of an ion in a quadrupole electric field, as expressed by the following equation (1), when the frequency f of the RF voltage is changed, an optimal voltage for an arbitrary mass-to-charge ratio must be changed by a ratio equal to the square of the frequency change.au=ax=−ay=4eU/(mω2r02)qu=qx=−qy=2eU/(mω2r02)  (1)For example, in the aforementioned case where the frequency f is increased by 0.2%, the optimal value of the V voltage or the U voltage will be the V voltage (or U voltage) at frequency f=1.20024 MHz multiplied by (1.20024/1.2)2. Accordingly, for an increase in the frequency f, if the V voltage is merely raised by the amount of decrease which accompanies the increase in the frequency to readjust the V voltage to its original level, a displacement of the m/z axis occurs. FIG. 10A is an example of the actual measurement in which the V voltage was readjusted from the state of FIG. 9B to the original level. A displacement of the m/z axis can be seen in the figure.
Furthermore, a displacement of the m/z axis also occurs when the U voltage is changed so as to maintain the U/V ratio at the same value. FIG. 10B is an example of the actual measurement which further included the step of adjusting the U voltage to bring the U/V ratio from the state of FIG. 10A back to the intended value. Again, a displacement of the m/z axis can be seen.
What is evident from the foregoing explanations is that, if the frequency-variable tuning method is adopted, it is necessary to adjust the mass-resolving power and the m/z axis by performing a manual adjustment or automatic tuning of the variable resistors 406 and 408 every time the frequency of the RF voltage is changed for the purpose of tuning